log, conf: Add support for logging to file

In some environments, such as KubeVirt pods, we might not have a
system logger available. We could choose to run in foreground, but
this takes away the convenient synchronisation mechanism derived from
forking to background when interfaces are ready.

Add optional logging to file with -l/--log-file and --log-size.

Unfortunately, this means we need to duplicate features that are more
appropriately implemented by a system logger, such as rotation. Keep
that reasonably simple, by using fallocate() with range collapsing
where supported (Linux kernel >= 3.15, extent-based ext4 and XFS) and
falling back to an unsophisticated block-by-block moving of entries
toward the beginning of the file once we reach the (mandatory) size
limit.

While at it, clarify the role of LOG_EMERG in passt.c.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.c

@@ -220,6 +220,7 @@ int main(int argc, char **argv)
 
 	__openlog(log_name, 0, LOG_DAEMON);
 
+	/* Meaning we don't know yet: log everything. LOG_EMERG is unused */
 	__setlogmask(LOG_MASK(LOG_EMERG));
 
 	c.epollfd = epoll_create1(EPOLL_CLOEXEC);